Various communication systems are known in the art. Pursuant to many such systems, an information signal is modulated on to a carrier signal and transmitted from a first location to a second location. At the second location, the information signal is demodulated and recovered.
Typically, the communication path used by such a system has various limitations, such as bandwidth. As a result, there are upper practical limitations that restrict the quantity of information that can be supported by the communication path over a given period of time. Various modulation schemes have been proposed that effectively increase the information handling capacity of the communication path as measured against other modulation techniques. For example, a 16 point quadrature amplitude modulation (QAM) approach provides a constellation of modulation values (distinguished from one another by phase and amplitude) wherein each constellation point represents a plurality of information bits.
Such QAM signals are typically transmitted in conjunction with a pilot component. For example, the information components of the QAM signal can be broadcast in conjunction with one or more pilot tones that are offset in frequency from the information content itself. These pilot components can be utilized to support synchronization, and to otherwise support recovery of the information component in a variety of ways.
Unfortunately, such frequency offset pilot components themselves consume bandwidth, thereby reducing the amount of bandwidth available in a communication path to support the information components. If the information components are themselves parsed into frequency offset data packages, the problem increases as further spectrum must be utilized to support the multiplicity of pilot references that are typically required to allow recovery of the various information packets.
In partial response to this situation, the prior art has proposed the use of time domain pilot components. For example, the information components of a particular QAM transmission are combined with an in band predetermined pilot reference component that appears in a periodic manner. (Since the pilot component appears only from time to time, the component is referred to as existing in the time domain, as distinguished from the frequency domain pilot components discussed above.)
Though suitable for many applications, QAM transmissions that include time domain pilot components are not satisfactory in all applications. For example, in an RF communication environment, where communication units may be in motion with respect to one another, such prior art time domain pilot reference QAM methodologies may provide unacceptable performance. In particular, the land-mobile radio channel is characterized by multipath fading that causes the channel phase and amplitude to vary over time as the receiving or transmitting unit moves about. Such variations must be compensated or otherwise allowed for in order to provide proper reception. Typically, phase and frequency modulation schemes avoid the need for compensation since channel amplitude variations can be ignored and differential or discriminator reception techniques can automatically account for the channel phase variations. However, phase and frequency modulation are not very bandwidth efficient. While QAM techniques can introduce bandwidth efficiency by comparison, QAM requires more complicated channel compensation methods, such as those prior art techniques that use one or more pilot tones in association with the information content.
Another problem associated with the multipath nature of the radio channel is that of frequency-selective fading. This occurs whenever the delay difference between the various multipath components that arrive at the receiver become large enough relative to the signalling rate in the channel. When this happens, the channel's frequency response will no longer appear to be flat in the band of interest, but will exhibit phase and amplitude variations with frequency, which in turn will vary with time as the transmitter or receiver moves about. This frequency-selective effect causes signal distortion that is present independent of the strength of the received signal. In data communication systems, this distortion manifests itself as an irreducible bit error rate, or error floor, that persists regardless of how strong the received signal becomes. In addition, the distortion effect worsens as the information capacity of the signal increases.
Accordingly, a need exists for a communication methodology that will provide efficient use of QAM (and the like) modulation techniques while simultaneously substantially avoiding spectral inefficiencies that may occur through use of certain prior art pilot component techniques and other multipath compensation techniques. This technique will preferably remain substantially robust in a varying multipath operating environment.